The grating spectrometer installation on the DIII‐D tokamak uses fundamental circular waveguide propagating the TE11 lowest‐order mode followed by oversized circular guide carrying the low‐loss TE01 mode. The short section of fundamental guide permits use of an electronic chopper operating at 100 kHz for both calibration and plasma operation. By using ac‐coupled amplifiers tuned to the chopping frequency, the background signal generated in the indium antimonide detectors by neutrons and x rays is automatically subtracted and the system noise bandwidth is reduced. Compared with a quasi‐optical system, the much smaller fundamental horn and front‐end waveguide allow the waveguide system to be located outside a gate valve. With this configuration the entire waveguide run, including the actual horn and vacuum window used during plasma operations, can be included in the calibration setup.